1. Field of the Invention
The present invention relates to a mixer circuit for mixing a plurality of input signals with each other, and more particularly, it relates to a mixer circuit having a high conversion gain, which is improved in linearity.
2. Description of the Background Art
FIG. 16 is a circuit diagram showing the structure of a conventional Gilbert cell type mixer circuit. Referring to FIG. 16, reference character 1G denotes a differential amplifier for amplifying an input voltage v1, and reference character 4A denotes a mixer part for mixing an output of the differential amplifier 1G with an input voltage v2. The conventional mixer circuit is formed by the mixer part 4A and the differential amplifier 1G, for outputting first and second output currents i1 and i2 from first and second output terminals 19 and 20 respectively.
The differential amplifier 1G is formed by an NPN bipolar transistor Q27 having a base which is connected to a non-inverting input terminal 150 and a collector which is connected to a first non-inverting input terminal 17 of the mixer part 4A and an emitter, an NPN bipolar transistor Q28 having a base which is connected to an inverting input terminal 151 and a collector which is connected to a first inverting input terminal 18 of the mixer part 4A and an emitter, current sources 152 and 153 for extracting prescribed direct currents IEE from a node which connects the emitters of the transistor Q27 and a resistor 154 and from a node which connects the emitter of the transistor Q28 and resistor 154 respectively, and the resistor 154 connecting the emitters of the transistors Q27 and Q28 with each other.
The mixer part 4A is formed by an NPN bipolar transistor Q3 having a base, a collector and an emitter which are connected to a second non-inverting input terminal 15, the first output terminal 19 and the first non-inverting input terminal 17 respectively, an NPN bipolar transistor Q4 having a base, a collector and an emitter which are connected to a second inverting input terminal 16, the second output terminal 20 and the first non-inverting input terminal 17 respectively, an NPN bipolar transistor Q5 having a base, a collector and an emitter which are connected to the second inverting input terminal 16, the first output terminal 19 and the first inverting input terminal 18 respectively, and an NPN bipolar transistor Q6 having a base, a collector and an emitter which are connected to the second non-inverting input terminal 15, the second output terminal 20 and the first inverting input terminal 18 respectively.
The inputted first input voltage v1 is amplified by an emitter differential pair circuit which is formed by the transistors Q27 and Q28, and appears as a difference between respective collector currents of the transistors Q27 and Q28. The respective collector currents of the transistors Q27 and Q28 define tail currents of an emitter differential pair circuit which is formed by the transistors Q3 and Q4 and that formed by the transistors Q5 and Q6. The second input voltage v2 is amplified by these emitter differential pair circuits formed by the transistors Q3, Q4, Q5 and Q6.
The aforementioned relations are expressed in the following numerical formulas. Ignoring the resistance value of the resistor 154, the respective collector currents ic27 and ic28 of the transistors Q27 and Q28 are expressed in the following numerical formulas 1 and 2:                               ic27          =                                    2              ⁢                              xe2x80x83                            ⁢              IEE                                      1              +                              exp                ⁡                                  [                                      -                                          v1                                              V                        T                                                                              ]                                                                    ⁢                  
                                    (        1        )                                ic28        =                              2            ⁢                          xe2x80x83                        ⁢            IEE                                1            +                          exp              ⁡                              [                                  -                                      v1                                          V                      T                                                                      ]                                                                        (        2        )            
Assuming that ic3, ic4, ic5 and ic6 represent respective collector currents of the transistors Q3 to Q6, these collector currents ic3 to ic6 are expressed in the following numerical formulas 3 to 6:                     ic3        =                  ic27                      1            +                          exp              ⁡                              [                                  -                                      v2                                          V                      T                                                                      ]                                                                        (        3        )                                ic4        =                  ic27                      1            +                          exp              ⁡                              [                                  v2                                      V                    T                                                  ]                                                                        (        4        )                                ic5        =                  ic28                      1            +                          exp              ⁡                              [                                  v2                                      V                    T                                                  ]                                                                        (        5        )                                ic6        =                  ic28                      1            +                          exp              ⁡                              [                                  -                                      v2                                          V                      T                                                                      ]                                                                        (        6        )            
From the numerical formulas 1 to 6, the collector currents ic3 to ic6 of the transistors Q3 to Q6 and the first and second input voltages v1 and v2 have relations of the following numerical formulas 7 to 10:                     ic3        =                              2            ⁢            IEE                                              [                              1                +                                  exp                  ⁡                                      [                                          -                                              v1                                                  V                          T                                                                                      ]                                                              ]                        ⁡                          [                              1                +                                  exp                  ⁡                                      [                                          -                                              v2                                                  V                          T                                                                                      ]                                                              ]                                                          (        7        )                                ic4        =                              2            ⁢            IEE                                              [                              1                +                                  exp                  ⁡                                      [                                          -                                              v1                                                  V                          T                                                                                      ]                                                              ]                        ⁡                          [                              1                +                                  exp                  ⁡                                      [                                          v2                                              V                        T                                                              ]                                                              ]                                                          (        8        )                                ic5        =                              2            ⁢            IEE                                              [                              1                +                                  exp                  ⁡                                      [                                          v1                                              V                        T                                                              ]                                                              ]                        ⁡                          [                              1                +                                  exp                  ⁡                                      [                                          v2                                              V                        T                                                              ]                                                              ]                                                          (        9        )                                ic6        =                              2            ⁢            IEE                                              [                              1                +                                  exp                  ⁡                                      [                                          v1                                              V                        T                                                              ]                                                              ]                        ⁡                          [                              1                +                                  exp                  ⁡                                      [                                          -                                              v2                                                  V                          T                                                                                      ]                                                              ]                                                          (        10        )            
From the numerical formulas 7 to 10, a differential output current (i1xe2x88x92i2) is given by the following numerical formula 11:                                                                         i1                -                i2                            =                              ic3                +                ic5                -                                  (                                      ic6                    +                    ic4                                    )                                                                                                        =                              2                ⁢                                                      IEE                    ⁡                                          [                                              tanh                        ⁡                                                  [                                                      v1                                                          2                              ⁢                                                              V                                T                                                                                                              ]                                                                    ]                                                        ⁡                                      [                                          tanh                      ⁡                                              [                                                  v2                                                      2                            ⁢                                                          V                              T                                                                                                      ]                                                              ]                                                                                                          (        11        )            
In general, tanhx can be expanded in a series as follows:                               tanh          ⁢                      xe2x80x83                    ⁢          x                =                  x          -                                    x              3                        3                                              (        12        )            
If x is sufficiently less than 1 in the numerical formula 11, the numerical formula 12 can be transformed into the following numerical formula 13, and hence the relation between the input voltages v1 and v2 and the output signals i1 and i2 is expressed in the following numerical formula 14:                               tanh          ⁢                      xe2x80x83                    ⁢          x                ≈        x                            (        13        )                                          i1          -          i2                ≈                  2          ⁢                                    IEE              ⁡                              [                                  v1                                      2                    ⁢                                          V                      T                                                                      ]                                      ⁡                          [                              v2                                  2                  ⁢                                      V                    T                                                              ]                                                          (        14        )            
Namely, this mixer circuit is adapted to multiply the first input voltage v1 by the second input voltage v2. If the first and second input voltages v1 and v2 are two signals having different frequencies f1 and f2, the mixer circuit multiplies these two signals by each other, thereby outputting signals having frequency components of the sum |f1+f2| of and the difference |f1xe2x88x92f2| between the frequencies of the two signals.
The emitters of the transistors Q27 and Q28 are connected with each other through the resistor 154, whereby a negative feedback effect results in the emitter differential pair circuit which is formed by the transistors Q27 and Q28. Namely, the conversion gain Gc of the mixer circuit is in inverse proportion to the resistance value of the resistor 154 forming a negative feedback circuit. The following numerical formula 15 expresses the relation between the conversion gain Gc and the resistance value RE of the resistor 154:                               G          c                ∝                  1          RE                                    (        15        )            
In order to increase the gain of the conventional mixer circuit having the aforementioned structure, the resistance value RE of the resistor 154 must be reduced. If the resistance value RE of the resistor 154 is reduced, however, nonlinearity of the circuit is disadvantageously increased to cause excess harmonics or intermodulation distortion.
According to a first aspect of the present invention, a mixer circuit comprises a mixer part having first and second input terminals and an output terminal for mixing first and second signals which are inputted from the first and second input terminals with each other, and an amplifier having an input terminal for receiving a third signal, an output terminal which is connected to the first input terminal, and a negative feedback circuit having a prescribed pass characteristic for passing the third signal with no filtration for feeding back an output signal, for amplifying the third signal and outputting the same to the mixer part as the first signal.
According to a second aspect of the present invention, the prescribed pass characteristic of the negative feedback circuit is a low-pass characteristic.
According to a third aspect of the present invention, the first input terminal of the mixer part comprises a first inverting input terminal and a first non-inverting input terminal, the second input terminal comprises a second inverting input terminal and a second non-inverting input terminal, the input terminal of the amplifier comprises an inverting input terminal for inputting the third signal and a non-inverting input terminal, and the amplifier is further provided with a first transistor having a control electrode which is connected to the non-inverting input terminal of the amplifier, a first current electrode which is connected to the first non-inverting input terminal, and a second current electrode, a second transistor having a control electrode which is connected to the inverting input terminal of the amplifier, a first current electrode which is connected to the first inverting input terminal, and a second current electrode, a first current source which is connected to the second current electrode of the first transistor for feeding a first direct current, a second current source which is connected to the second current electrode of the second transistor for feeding a second direct current, and a low-pass filter which is connected between the second current electrodes of the first and second transistors.
According to a fourth aspect of the present invention, the first input terminal of the mixer part comprises a first inverting input terminal and a first non-inverting input terminal, the second input terminal comprises a second inverting input terminal and a second non-inverting input terminal, the input terminal of the amplifier comprises an inverting input terminal for inputting the third signal and a non-inverting input terminal, and the amplifier is further provided with a first transistor having a control electrode which is connected to the non-inverting input terminal of the amplifier, a first current electrode which is connected to the first non-inverting input terminal, and a second current electrode, a second transistor having a control electrode which is connected to the inverting input terminal of the amplifier, a first current electrode which is connected to the first inverting input terminal, and a second current electrode, a first low-pass filter having a first end which is connected to the second current electrode of the first transistor, and a second end, a second low-pass filter having a first end which is connected to the second current electrode of the second transistor, and a second end which is connected to the second end of the first low-pass filter, and a current source which is connected to the second end of the first low-pass filter for feeding a prescribed direct current.
According to a fifth aspect of the present invention, the prescribed pass characteristic of the negative feedback circuit is a bandpass characteristic.
According to a sixth aspect of the present invention, the first input terminal of the mixer circuit comprises a first inverting input terminal and a first non-inverting input terminal, the second input terminal comprises a second inverting input terminal and a second non-inverting input terminal, the input terminal of the amplifier comprises an inverting input terminal for inputting the third signal and a non-inverting input terminal, and the amplifier is further provided with a first transistor having a control electrode which is connected to the non-inverting input terminal of the amplifier, a first current electrode which is connected to the first non-inverting input terminal, and a second current electrode, a second transistor having a control electrode which is connected to the inverting input terminal of the amplifier, a first current electrode which is connected to the first inverting input terminal, and a second current electrode, a first bandpass filter having a first end which is connected to the second current electrode of the first transistor, and a second end, a second bandpass filter having a first end which is connected to the second current electrode of the second transistor, and a second end which is connected to the second end of the first bandpass filter, and a current source which is connected to the second end of the first bandpass filter for feeding a prescribed direct current.
According to a seventh aspect of the present invention, the first input terminal of the mixer part comprises a first inverting input terminal and a first non-inverting input terminal, the second input terminal comprises a second inverting input terminal and a second non-inverting input terminal, the input terminal of the amplifier comprises an inverting input terminal for inputting the third signal and a non-inverting input terminal, and the amplifier is further provided with a first transistor having a control electrode which is connected to the non-inverting input terminal of the amplifier, a first current electrode which is connected to the first inverting input terminal, and a second current electrode, a second transistor having a control electrode which is connected to the inverting input terminal of the amplifier, a first current electrode which is connected to the first non-inverting input terminal, and a second current electrode, a first current source which is connected to the second current electrode of the first transistor for feeding a first direct current, a second current source which is connected to the second current electrode of the second transistor for feeding a second direct current, and a bandpass filter which is connected between the second current electrodes of the first and second transistors.
According to an eighth aspect of the present invention, the negative feedback circuit comprises an inductor.
According to a ninth aspect of the present invention, a mixer circuit comprises a mixer part having first and second input terminals and an output terminal, as well as a negative feedback circuit having a prescribed pass characteristic for passing signals which are inputted from the first and second input terminals with no filtration for negatively feeding back processing paths for the signals which are inputted from the first and second input terminals, for mixing first and second signals inputted from the first and second input terminals with each other, and an amplifier having an input terminal for receiving a third signal and an output terminal which is connected to the first input terminal, for amplifying the third signal and outputting the same to the mixer part as the first signal.
According to a tenth aspect of the present invention, the prescribed pass characteristic of the negative feedback circuit is a low-pass characteristic.
According to an eleventh aspect of the present invention, the first input terminal of the mixer part comprises a first inverting input terminal and a first non-inverting input terminal, the second input terminal comprises a second inverting input terminal and a second non-inverting input terminal, the output terminal of the mixer part comprises first and second output terminals, and the mixer part is further provided with a first transistor having a control electrode which is connected to the second non-inverting input terminal, a first current electrode which is connected to the first output terminal, and a second current electrode, a second transistor having a control electrode which is connected to the second inverting input terminal, a first current electrode which is connected to the second output terminal, and a second current electrode, a first low-pass filter having a first end which is connected to the second current electrode of the first transistor, and a second end which is connected to the first inverting input terminal, a second low-pass filter having a first end which is connected to the second current electrode of the second transistor, and a second end which is connected to the first inverting input terminal, a third transistor having a control electrode which is connected to the second inverting input terminal, a first current electrode which is connected to the first output terminal, and a second current electrode, a fourth transistor having a control electrode which is connected to the second non-inverting input terminal, a first current electrode which is connected to the second output terminal, and a second current electrode, a third low-pass filter having a first end which is connected to the second current electrode of the third transistor, and a second end which is connected to the first non-inverting input terminal, and a fourth low-pass filter having a first end which is connected to the second current electrode of the fourth transistor, and a second end which is connected to the first non-inverting input terminal.
According to a twelfth aspect of the present invention, the prescribed pass characteristic of the negative feedback circuit is a bandpass characteristic.
According to a thirteenth aspect of the present invention, the first input terminal of the mixer part comprises a first inverting input terminal and a first non-inverting input terminal, the second input terminal comprises a second inverting input terminal and a second non-inverting input terminal, the output terminal of the mixer part comprises first and second output terminals, and the mixer part is further provided with a first transistor having a control electrode which is connected to the second non-inverting input terminal, a first current electrode which is connected to the first output terminal, and a second current electrode, a second transistor having a control electrode which is connected to the second inverting input terminal, a first current electrode which is connected to the second output terminal, and a second current electrode, a first bandpass filter having a first end which is connected to the second current electrode of the first transistor, and a second end which is connected to the first inverting input terminal, a second bandpass filter having a first end which is connected to the second current electrode of the second transistor, and a second end which is connected to the first inverting input terminal, a third transistor having a control electrode which is connected to the second inverting input terminal, a first current electrode which is connected to the first output terminal, and a second current electrode, a fourth transistor having a control electrode which is connected to the second non-inverting input terminal, a first current electrode which is connected to the second output terminal, and a second current electrode, a third bandpass filter having a first end which is connected to the second current electrode of the third transistor, and a second end which is connected to the first non-inverting input terminal, and a fourth bandpass filter having a first end which is connected to the second current electrode of the fourth transistor, and a second end which is connected to the first non-inverting input terminal.
According to a fourteenth aspect of the present invention, the low-pass filter comprises a secondary or higher order low-pass filter.
According to a fifteenth aspect of the present invention, the low-pass filter comprises a series body comprising an inductor and a resistor which are connected between the second current electrodes of the first and second transistors.
According to a sixteenth aspect of the present invention, the bandpass filter comprises a secondary or higher order bandpass filter.
In the mixer circuit according to the first aspect of the present invention, as hereinabove described, the negative feedback circuit of the amplifier has the prescribed pass characteristic for passing the third signal with no filtration, whereby a high conversion gain is attained while an unnecessary frequency component is reduced in amplification so that intermodulation distortion having a frequency of the sum of or the difference between the two signals inputted from the first and second input terminals, for example, is reduced and distortion of the output of the mixer circuit can be effectively reduced.
In the mixer circuit according to the second aspect of the present invention, the negative feedback circuit of the amplifier has the low-pass characteristic, whereby a harmonic component can be reduced and distortion of the output of the mixer circuit can be effectively suppressed.
In the mixer circuit according to the third aspect of the present invention, the feedback amount of the differential pair circuit which is formed by the first and second transistors can be increased by the low-pass filter which is connected between the second current electrodes of the first and second transistors forming the differential pair circuit as the frequency is increased, whereby a mixer circuit having a high conversion gain and small output distortion can be effectively obtained in a simple structure.
In the mixer circuit according to the fourth aspect of the present invention, the negative feedback of the differential pair circuit which is formed by the first and second transistors can be decided by the first and second low-pass filters which are serially connected between the second current electrodes of the first and second transistors forming the differential pair circuit, whereby a mixer circuit having a high conversion gain and small output distortion can be effectively obtained in a simple structure.
In the mixer circuit according to the fifth aspect of the present invention, the negative feedback circuit of the amplifier has the bandpass characteristic, whereby unnecessary frequency components other than the frequencies of the input signals can be reduced and output distortion of the mixer circuit can be effectively reduced.
In the mixer circuit according to the sixth aspect of the present invention, the feedback amount of the differential pair circuit which is formed by the first and second transistors can be increased by the first and second bandpass filters which are connected between the second current electrodes of the first and second transistors forming the differential pair circuit as the frequency component is unnecessary, whereby a mixer circuit having a high conversion gain and small output distortion can be effectively obtained in a simple structure.
In the mixer circuit according to the seventh aspect of the present invention, the feedback amount of the differential pair circuit which is formed by the first and second transistors can be increased by the bandpass filter which is connected between the second current electrodes of the first and second transistors forming the differential pair circuit as the frequency component is unnecessary, whereby a mixer circuit having a high conversion gain and small output distortion can be effectively obtained in a simple structure.
In the mixer circuit according to the eighth aspect of the present invention, a filter having a prescribed pass characteristic can be brought into a simple structure, whereby an excellent characteristic can be easily attained.
In the mixer circuit according to the ninth aspect of the present invention, the mixer part has the prescribed pass characteristic for passing the first signal with no filtration with respect to the signal inputted from the first input terminal, whereby a high conversion gain is attained while an unnecessary frequency component is reduced in amplification so that intermodulation distortion having a frequency of the sum of or the difference between the two signals inputted from the first and second input terminals, for example, is reduced and distortion of the output of the mixer circuit can be effectively reduced.
In the mixer circuit according to the tenth aspect of the present invention, the negative feedback circuit in the mixer part has the low-pass characteristic, whereby a harmonic component can be reduced in mixing of the signals and distortion of the output of the mixer circuit can be effectively reduced.
In the mixer circuit according to the eleventh aspect of the present invention, the negative feedback of the differential pair circuit which is formed by the first and second transistors can be decided by the first and second low-pass filters which are serially connected between the second current electrodes of the first and second transistors and the negative feedback of the differential pair circuit which is formed by the third and fourth transistors can be decided by the third and fourth low-pass filters which are serially connected between the second current electrodes of the third and fourth transistors, whereby a mixer circuit having a high conversion gain and small output distortion can be effectively obtained in a simple structure.
In the mixer circuit according to the twelfth aspect of the present invention, the negative feedback circuit of the mixer part has the bandpass characteristic, whereby unnecessary frequency components other than the frequencies of the input signals can be reduced and output distortion of the mixer circuit can be reduced.
In the mixer circuit according to the thirteenth aspect of the present invention, the negative feedback of the differential pair circuit which is formed by the first and second transistors can be decided by the first and second bandpass filters which are serially connected between the second current electrodes of the first and second transistors and the negative feedback of the differential pair circuit which is formed by the third and fourth transistors can be decided by the third and fourth bandpass filters which are serially connected between the second current electrodes of the third and fourth transistors, whereby a mixer circuit having a high conversion gain and small output distortion can be effectively obtained in a simple structure.
In the mixer circuit according to the fourteenth aspect of the present invention, a higher order low-pass filter has higher ability for eliminating high frequencies, whereby the performance for improving output nonlinearity can be improved as compared with the case of employing a primary order low-pass filter.
In the mixer circuit according to the fifteenth aspect of the present invention, the inductor and the resistor are connected in series with each other, whereby the occupied area can be reduced and a mixer circuit suitable for an integrated circuit can be effectively obtained.
In the mixer circuit according to the sixteenth aspect of the present invention, a higher bandpass filter has higher ability for eliminating unnecessary frequency components, whereby output distortion can be reduced as compared with the case of employing a primary low-pass filter.
Accordingly, an object of the present invention is to obtain a mixer circuit having a high conversion gain, which is excellent in linearity.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.